Technical Field
The present invention relates to a variable capacitance device and an antenna device that utilizes the variable capacitance device.
Background Art
In NFC (near field communication) modules used for mobile FeliCa, a phenomenon has been known to occur in which reception sensitivity decreases as a result of the resonant frequency shifting away from 13.56 MHz due to variations in the antenna coil, for example. To correct these shifts in the resonant frequency, frequency adjustment circuits that include capacitors are placed inside the modules, all parts are inspected before the modules are shipped, and the capacitance of the capacitors is minutely adjusted.
Conventionally, switched capacitors, in which FET (field effect transistor) switches are connected in series in a fixed capacitance element, have been used. Switchover settings are then written onto a control IC (integrated circuit) during the pre-shipping inspection, and when NFC is being used, the module switches over to the FET mode and adjusts the capacitance of the capacitor.
However, general-purpose ceramic capacitors, which have become cheaper than FET switches in recent years and which can handle high voltages, have been considered as a possible replacement for FET switches. Ceramic capacitor materials have a property that has been actively utilized: the capacitance decreases as a DC bias voltage is applied.
The capacitance of ceramic capacitors changes over time, however, which is a problem. This has led to consideration of using variable capacitance devices that utilize a plurality of variable capacitance elements that include a dielectric formed via a thin film instead of by sintering.
Since conventional variable capacitance devices have directionality due to their structure, however, there is the possibility that if the capacitor is mounted incorrectly, an adequate change in capacitance will not be achieved when voltage is applied.
FIGS. 1A and 1B show an example configuration of a conventional variable capacitance device. In the conventional variable capacitance device, capacitors C101 to C104 are connected in series between the input terminal IN and the output terminal OUT, and terminals X and Y, which are used for applying bias voltage, are provided respectively on the left side and the right side of the device, for example. As shown in FIG. 1A, the correct configuration (also referred to as a “positive connection”) is to connect the terminal X, which is connected to the variable capacitance elements C101 to C104 via three resistors, to the ground GND and to then apply a prescribed voltage DC+ to the terminal Y, which is connected to the variable capacitance elements C101 to C104 via two resistors. Current flows from the terminal Y to the terminal X, as shown by the arrows.
FIG. 1B, on the other hand, shows an incorrect configuration (known as a “reverse connection”) in which the terminal Y is connected to the ground GND, and the prescribed voltage DC+ is applied to the terminal X. In such a case, current flows from the terminal X to the terminal Y, as shown by the arrows, which means that current does not flow to the variable capacitance elements C101 and C104 and there is no change in the applied voltage.
As shown in FIG. 2, in a positive connection, the capacitance of each of the variable capacitance elements C101 to C104 is 400 nF when DC+=0V, and the capacitance of each of the capacitance elements decreases by 33% to 268 nF when DC+=+3V, for example. This means that the overall capacitance is 100 nF when DC+=0V and 67 nF when DC+=+3V, which means that the overall capacitance also changes by 33%.
In a reverse connection, on the other hand, the capacitance of the variable capacitance elements C102 and C103 decreases by 33% to 268 nF when DC+=+3V, but the capacitance of the variable capacitance elements C101 and C104 does not change. Therefore, the overall capacitance is 100 nF when DC+=0V and 80 nF when DC+=+3V, which means that the overall capacitance only changes by 20%.
This means that there will be instances in which the capacitance of the capacitor cannot be properly adjusted and variations in the resonant frequency cannot be fully corrected.